Given these large ranges of future emissions and their driving forces, there
is an infinite number of possible alternative futures to explore. The SRES scenarios
cover a finite, albeit a very wide range, of future emissions. The approach
involved the development of a set of four alternative scenario "families" comprising
40 SRES scenarios subdivided into seven scenario groups. During the approval
process of the SPM in March 2000 at the 5th Session of WGIII in Katmandu, it
was decided to combine two of these groups into one, resulting in six groups.
To facilitate the process of identifying and describing alternative future developments,
each scenario family includes a coherent narrative part called a "storyline,"
and a number of alternative interpretations and quantifications of each storyline
developed by six different modeling approaches. All the interpretations and
quantifications of one storyline together are called a scenario family (see
also Box 1-1 in Chapter 1 on terminology). Each storyline describes a demographic,
social, economic, technological, environmental, and policy future for one of
these scenario families. Storylines were formulated by the writing team in a
process which identified driving forces, key uncertainties, possible scenario
families, and their logic. Within each family different scenarios explore variations
of global and regional developments and their implications for GHG and sulfur
emissions. Each of these scenarios is consistent with the broad framework of
that scenario family as specified by the storyline. Consequently, each scenario
family and scenario group is equally sound. Chapters 4 and 5 give a more detailed
description of the storylines, their quantifications, and the resultant 40 emissions
scenarios.

The main reasons for formulating storylines are:

to help the writing team to think more coherently about the complex interplay
among scenario driving forces within each and across alternative scenarios;

to make it easier to explain the scenarios to the various user communities
by providing a narrative description of alternative futures that goes beyond
quantitative scenario features;

to make the scenarios more useful, in particular to analysts who contribute
to IPCC WGII and WGIII; the social, political, and technological context described
in the scenario storylines is all-important in analyzing the effects of policies
either to adapt to climate change or to reduce GHG emissions; and

to provide a guide for additional assumptions to be made in detailed climate
impact and mitigation analyses, because at present no single model or scenario
can possibly respond to the wide variety of informational and data needs of
the different user communities of long-term emissions scenarios.

The SRES writing team reached broad agreement that there could be no "best
guess" scenarios; that the future is inherently unpredictable and that views
will differ on which of the storylines could be more likely. The writing team
decided on four storylines: an even number helps to avoid the impression that
there is a "central" or "most likely" case. The team wanted more than two in
order to help illustrate that the future depends on many different underlying
dynamics; the team did not want more than four, as they wanted to avoid complicating
the process by too many alternatives. There is no "business-as-usual" scenario.
Nor should the scenarios be taken as policy recommendations. The storylines
represent the playing out of certain social, economic, technological, and environmental
paradigms, which will be viewed positively by some people and negatively by
others. The scenarios cover a wide range, but not all possible futures. In particular,
it was decided that possible "surprises" would not be considered and that there
would be no "disaster" scenarios that are difficult to quantify with the aid
of formal models.

The titles of the storylines have been kept simple: A1, A2, B1, and B2. There
is no particular order among the storylines; Box TS-1 lists them in alphabetic
order. Figure TS-2 schematically illustrates the four
storylines and scenario families. Each is based on a common specification of
the main driving forces. They are shown, very simplistically, as branches of
a two-dimensional tree. The two dimensions shown indicate the global-regional
and the development-environmental orientation, respectively. In reality, the
four scenario families share a space of a much higher dimensionality given the
numerous driving forces and other assumptions needed to define any given scenario
in a particular modeling approach. The team decided to carry out sensitivity
tests within some of the storylines by considering alternative scenarios with
different fossil-fuel reserves, rates of economic growth, or rates of technical
change within a given scenario family. For example, four scenario "groups" within
the A1 scenario family were explored. As mentioned, two of these four groups
that explore fossil-intensive developments in the energy system were merged
in the SPM. Together with the other three scenario families this results in
seven groups of scenarios effectively six equally sound groups after the merging
of the two fossil-intensive groups in the SPM - that share common assumptions
of some of the key driving forces and are thus not independent of each other.

Figure TS-2: Schematic illustration
of SRES scenarios. The four scenario "families" are shown, very simplistically,
as branches of a two-dimensional tree. In reality, the four scenario families
share a space of a much higher dimensionality given the numerous assumptions
needed to define any given scenario in a particular modeling approach. The
schematic diagram illustrates that the scenarios build on the main driving
forces of GHG emissions. Each scenario family is based on a common specification
of some of the main driving forces. The A1 storyline branches out into four
groups of scenarios to illustrate that alternative development paths are
possible within one scenario family. Two of these groups were merged in
the SPM.

All four storylines and scenario families describe future worlds that are generally
more affluent compared to the current situation. They range from very rapid
economic growth and technological change to high levels of environmental protection,
from low to high global populations, and from high to low GHG emissions. What
is perhaps even more important is that all the storylines describe dynamic changes
and transitions in generally different directions. Although they do not include
additional climate initiatives, none of them are policy free. As time progresses,
the storylines diverge from each other in many of their characteristic features.
In this way they allow us to span the relevant range of GHG emissions and different
combinations of their main sources.

Box TS-1: The Main Characteristics of the Four SRES Storylines and
Scenario Families
By 2100 the world will have changed in ways that are hard to imagine -
as hard as it would have been at the end of the 19th century to imagine
the changes of the 100 years since. Each storyline assumes a distinctly
different direction for future developments, such that the four storylines
differ in increasingly irreversible ways. Together they describe divergent
futures that encompass a significant portion of the underlying uncertainties
in the main driving forces. They cover a wide range of key "future" characteristics
such as population growth, economic development, and technological change.
For this reason, their plausibility or feasibility should not be considered
solely on the basis of an extrapolation of current economic, technological,
and social trends.

The A1 storyline and scenario family describes a future world of very
rapid economic growth, low population growth, and the rapid introduction
of new and more efficient technologies. Major underlying themes are
convergence among regions, capacity building, and increased cultural
and social interactions, with a substantial reduction in regional differences
in per capita income. The A1 scenario family develops into four groups
that describe alternative directions of technological change in the
energy system. Two of the fossil-intensive groups were merged in the
SPM.

The A2 storyline and scenario family describes a very heterogeneous
world. The underlying theme is self-reliance and preservation of local
identities. Fertility patterns across regions converge very slowly,
which results in high population growth. Economic development is primarily
regionally oriented and per capita economic growth and technological
change are more fragmented and slower than in other storylines.

The B1 storyline and scenario family describes a convergent world
with the same low population growth as in the A1 storyline, but with
rapid changes in economic structures toward a service and information
economy, with reductions in material intensity, and the introduction
of clean and resource-efficient technologies. The emphasis is on global
solutions to economic, social, and environmental sustainability, including
improved equity, but without additional climate initiatives.

The B2 storyline and scenario family describes a world in which the
emphasis is on local solutions to economic, social, and environmental
sustainability. It is a world with moderate population growth, intermediate
levels of economic development, and less rapid and more diverse technological
change than in the B1 and A1 storylines. While the scenario is also
oriented toward environmental protection and social equity, it focuses
on local and regional levels.

After determining the basic features and driving forces for each of the
four storylines, the team began modeling and quantifying the storylines.
This resulted in 40 scenarios, each of which constitutes an alternative
interpretation and quantification of a storyline. All the interpretations
and quantifications associated with a single storyline are called a scenario
family (see Chapter 1 for terminology and Chapter
4 for further details).